RTC makes use of DNS SRV records for load-balancing and failover. A key feature of the SRV record is that the TCP or UDP port number is specified in the record. Table 5.1, “DNS records for the example” demonstrates the use of standard port numbers for SIP, TURN and XMPP.

If users are connecting to the service from arbitrary locations, including public wi-fi hotspots, hotels and the offices of other companies, they will almost certainly encounter firewalls that only allow traffic to pass on a limited range of port numbers or through HTTP proxy servers.

For this reason, it is common to operate RTC services on port 443 instead of the normal port numbers. Two or more processes can't listen on the same port number on the same IP address. When all the RTC processes have to use port 443, it is necessary to have a different IP address for each process. Table 5.2, “Protocols using port 443” gives a summary of the ports to change.